On Thu, Jun 09, 2005 at 08:48:22AM -0700, Ms Katie Kelly wrote:
> So, if you put these fuel saving tires on a Hummer, it would burn
> significantly less gas? That would be a miracle.
My former housemate, who was neurotic about gas mileage, did notice
the difference between OE tires and replacement tires in terms of gas
mileage. This was on her Geo Prizm (a Corrolla with funny bumpers)
which gets nearly 40 miles per gallon the way she drives it. She
noticed about a 1-2 MPG drop when she got the new tires. She
researched what was going on and found that OE tires are generally
optimized for high fuel economy, and aftermarket tires are not. You
can find an example of this in the Subaru WRX, which came with the
same Bridgestone RE92 low rolling resistance tires that were used on
the Honda Insight. Anyone who has autocrossed a WRX on the OE tires
knows the tradeoffs (I have not, but I've sure heard about it and seen
the results).
So, if these tires can gain 2MPG on a Corrolla, we can save a lot of
gas, right? Not really. Rolling resistance is a fairly small loss of
energy, and only comes into play if the air resistance and drivetrain
losses are small. Reducing rolling resistance will have less of an
effect on vehicles that get lousy gas mileage, because those vehicles
aren't burning all that gas to overcome rolling resistance. They're
burning it to push a large, non-aerodynamic shape through the air.
They're burning it to keep their beefy engines and transmissions
spinning. The older cars are wasting it on inefficient combustion and
blowing a lot of it unburned out the tailpipe.
In the big picture, it's the cars with poor fuel economy that burn
most of the gas. Consider: if half the cars got 10 MPG and half the
cars got 40 MPG and they drove an equal number of miles, the 10 MPG
cars would be using 80% of the gas. The average gas mileage for all
cars would be 16 MPG.
Perhaps you could improve the mileage of the 40MPG cars to 42MPG using
low rolling resistance tires. Suppose that the 10MPG cars can save
twice as much energy by switching tires (they are heavier and use
bigger tires, so their base rolling resistance is probably higher).
That gives them 10.24 MPG. Combined for all cars is now 16.5 MPG,
about a 3% change.
Now that was just a bunch of hand waving and assumptions, but it's
probably good enough to be called a government study, don't you think?
--
john@idsfa.net John Stimson
http://www.idsfa.net/~john/ HMC Physics '94
|